An active matrix substrate for use in liquid crystal display devices, or the like, includes a switching element, such as a thin film transistor (hereinafter, “TFT”), in each pixel. Known examples of such a switching element include a TFT in which an oxide semiconductor layer is used as the active layer (hereinafter, referred to as “oxide semiconductor TFT”). Patent Document 1 discloses a liquid crystal display device in which InGaZnO (an oxide including indium, gallium and zinc) is used in the active layer of the TFT.
The oxide semiconductor TFT is capable of operating at a higher speed than the amorphous silicon TFT. The oxide semiconductor film is manufactured through a simpler process than the polycrystalline silicon film and is therefore applicable to devices which require a large area. Thus, the oxide semiconductor TFT is a promising candidate for a high-performance active element which can be manufactured with reduced manufacturing steps and a reduced manufacturing cost.
Since the mobility of the oxide semiconductor is high, it is possible to achieve equal or higher performance even if the size is reduced as compared with conventional amorphous silicon TFTs. Therefore, when an active matrix substrate of a display device is manufactured using the oxide semiconductor TFT, the area occupancy of the TFT in each pixel can be reduced, and the pixel aperture ratio can be improved. Accordingly, bright display can be realized even with a reduced amount of light from the backlight, and low power consumption can be realized.
Since the off-leak characteristic of the oxide semiconductor TFT is excellent, it is possible to utilize an operation mode in which display is performed with reduced image rewriting frequency. For example, in the case of displaying a still image, it is possible to operate the TFT so as to rewrite the image data once every second. Such a driving method is referred to as “intermittent driving” or “low frequency driving” and is capable of greatly reducing the power consumption by the display device.